Blood pressure measurement is used to generally indicate the health of a patient or the type of emergency treatment which is necessary for an accident victim or a battlefield casualty. Most blood pressure measurement systems involve listening for blood pulses of a patient with a stethoscope. Noises of an emergency vehicle or in a battlefield make these pulses difficult to hear, which makes accurate measurement of blood pressure very difficult to obtain.
FIG. 1 shows a schematic diagram of a blood pressure measurement system that determines systolic, diastolic, and mean arterial pressures without detecting blood pulses. The blood pressure measurement system of FIG. 1 is particularly useful in emergency vehicles and battlefield situations, where movement and noise drastically affect accurate measurement of blood pressure in systems that rely on the detection of blood pulses. The system of FIG. 1 filters out blood pulses and high frequency noise, and processes signals having frequencies on the order of 1 Hz.
The operation of the blood pressure measurement system of FIG. 1 is detailed in U.S. Pat. No. 4,649,928 to Samaras et al., which has been assigned to the same entity as this invention. This application incorporates the disclosure of U.S. Pat. No. 4,649,928 by reference.
FIG. 1 shows an occlusion bladder 10 on the upper arm 8 of a patient, which is inflated to a pressure substantially higher than systolic blood pressure of the patient to stop blood flow in the upper limb 8. A lower sensing bladder 12 on the forearm 6 of the patient is inflated to a nominal value, such as 70 mmHg. Pressure in the occlusion bladder 10 is gradually decreased over a period of approximately thirty seconds. A microprocessor 28 controls valves 16, 18, and 20 and controls pump 14 to inflate and deflate the occlusion bladder 10 and the sensing bladder 12. Transducers 22 and 24 monitor pressures in the occlusion bladder 10 and the sensing bladder 12, respectively. These transducers 22 and 24 respectively produce an occlusion pressure signal and a sensing pressure signal which are received by a microprocessor 28. The microprocessor 28 filters out pulses corresponding to a patient's heart beat from the sensing pressure signal produced by the transducer 24.
An occlusion pressure value sensed by the transducer 22 on the occlusion bladder 10 corresponds to systolic pressure of a patient when the sensing pressure signal from the sensing bladder 12 reaches a minimum value. The microprocessor 28 derives first, second, and third time derivatives of the sensing pressure signal from the sensing bladder 12. An occlusion pressure value from the occlusion bladder 10 corresponds to mean arterial pressure when the third time derivative of the sensing pressure signal is a positive-going, zero-crossing signal. The microprocessor 28 derives diastolic pressure from the mean arterial pressure and the systolic pressure according to a well known relationship.
The present inventors have found that the system of FIG. 1 is somewhat responsive to very low-frequency, high-amplitude noise signals, which can cause the system of FIG. 1 to produce false values of systolic, mean arterial, and diastolic pressures. These noise signals are produced when a patient wearing the system is slowly transported by an emergency vehicle over a large bump, for instance. These noise signals are also produced when a patient wearing the system makes slow, large movements with his fingers, for instance. In an emergency situation, it is critical to accurately measure these pressures for correct treatment of a patient. Thus, a need exists for determining the presence of such slow, large noise signals and for filtering out these noise signals to accurately 10 measure blood pressure of a patient.